you need one set for the usual US teams (OD, woodland, digital or multicam) and you need one set for the usual rebels (anything not mentionned above like DCU, flecktarn or DPM)
keep in mind that desert tones are more easily seen in the forest ( DCU, ACU, beige and multicam ) and that's also why I never use black.
for instance I use OD and woodland for when I play US,
Tiger+DPM when I'm rebel.( I just bought tiger pants
)